RU2780617C1 - Rail - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to a rail used on freight railways. The rail is made of steel, including, wt. %: C 0.75-1.20, Si 0.10-2.00, Mn 0.10-2.00, Cr 0-2,00, Mo 0-0,50, Co 0-1,00, B 0-0,0050, Cu 0-1,00, Ni 0-1,00, V 0-0,50, Nb 0-0,050, Ti 0-0,0500, Mg 0-0,0200, Ca 0-0,0200, REM 0-0,0500, Zr 0-0,0200, N 0-0,0200, Al 0-1,00, P 0.0250 or less, S 0.0250 or less and a residue of iron and impurities in which 90% of the area or more of the metallographic structure in the cross section of the rail neck part is a perlite structure. The minimum hardness value in the cross section of the rail neck part is 300 or more HV, the difference between the maximum and the minimum hardness values in the cross section of the rail neck part is 40 or less HV. The specified cross-section of the part of the rail neck represents an area in the range of +/-15 mm in the up-down direction of the rail from the midline between the upper part of the rail and the lower part of the rail, and the mentioned hardness was measured with a Vickers hardness tester with a load of 98 H, while the recesses were continuously performed in the direction of the thickness of the part of the rail neck in a row with a step of 1.0 mm, in this case, the starting point of continuous indentation is the position at a depth of 1.0 mm from the outer surface of the part of the rail neck, and the hardness measurement was performed on at least five lines.

EFFECT: suppression of fatigue damage arising from a part of the rail neck is provided, rail has a high resistance to fatigue failure.

3 cl, 5 dwg, 4 tbl

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

[0001]

The present invention relates to a rail which is used in freight railways and has excellent damage resistance.

Priority is claimed from Japanese Patent Application No. 2019-048809, filed March 15, 2019, the contents of which are hereby incorporated by reference.

PRIOR ART

[0002]

In the process of economic development, new natural resources such as coal are developed. In particular, the extraction of natural resources has accelerated in underdeveloped regions with harsh natural environments. Along with this, the environment around the railroad tracks used to transport resources has become much harsher. For this reason, rails must now have greater wear resistance than ever before. Hence there is a need to develop rails with improved wear resistance.

[0003]

In addition, in recent years, rail traffic has become more stressed, and fatigue damage can occur from the neck part of the rail. For this reason, in order to further increase the service life of the rail, there is a need to improve the fatigue damage resistance of the rail neck part in addition to improving the wear resistance of the head part. This need is especially important for rails used on curved tracks. In the case of a curved track, it is clear from recent studies that since the stress towards the outside of the curve is applied to the rail head part, causing a bending stress to be applied to the rail neck part, fatigue damage may occur from the neck part as the starting point.

[0004]

In order to improve the wear resistance of rail steel, high-strength rails have been developed as described in Patent Documents 1 and 2, for example. heat treatment, or an increase in the volume fraction of cementite in the pearlite plates of the rail head part due to an increase in the amount of carbon in the steel.

[0005]

Patent Document 1 discloses that a rail having excellent wear resistance can be obtained by performing accelerated cooling of a portion of the rail head that is rolled or reheated at a cooling rate of 1-4°C/s from an austenite temperature range to a range of 850-500°C. .

[0006]

In addition, Patent Document 2 discloses that a rail having excellent wear resistance can be obtained by increasing the volume fraction of cementite in the pearlite structure laminations of the rail head portion using hypereutectoid steel (C: more than 0.85% and 1.20% or less).

[0007]

In the technique disclosed in Patent Documents 1 or 2, by increasing the hardness by reducing the lamellar spacing in the pearlite structure of the rail head portion or by increasing the volume fraction of cementite in the lamellar structure of the pearlite, the wear resistance of the rail head portion is improved and the service life is extended to some extent. However, in the rails disclosed in Patent Documents 1 and 2, no studies have been conducted on fatigue damage resistance, which prevents fatigue damage to the rail neck portion.

[0008]

In addition, for example, Patent Document 3 discloses that a rail having improved toughness in the rail neck portion can be obtained by controlling the amount of pro-eutectoid (secondary) cementite structure formed in the rail neck portion.

[0009]

In the technique disclosed in Patent Document 3, the amount of cementite structure formed in the pearlite structure is controlled to improve the toughness of the rail neck portion, suppress rail failure, and to some extent increase the service life. However, in the rail disclosed in Patent Document 3, no research has been conducted on fatigue damage resistance, which prevents fatigue damage to the rail neck part.

[0010]

In addition, for example, Patent Document 4 discloses that a rail having improved fatigue properties of the rail neck portion can be obtained by reducing the residual stress by cooling the weld portion of the rail immediately after welding.

[0011]

In the technique disclosed in Patent Document 4, the residual stress of the rail weld portion is adjusted to improve the fatigue properties of the rail neck portion, prevent rail breakage, and to some extent increase the service life. However, in the rail disclosed in Patent Document 4, the purpose is to weld the rail, and no research has been done to prevent fatigue damage to the base material of the rail. In addition, in the technique disclosed in Patent Document 4, residual stress is controlled, and in Patent Document 4, no research has been done on the relationship between material and hardness and fatigue properties of the rail neck part.

[0012]

In addition, in the method disclosed in Patent Document 5, in the rail heat treatment method, the hardness of the rail neck portion required to provide the toughness is determined. However, in the rail disclosed in Patent Document 5, no research has been carried out to prevent fatigue damage to the rail neck part. In addition, Patent Document 5 only illustrates the range of the average hardness of the neck part, and no study has been made of the hardness distribution that affects the fatigue damage suppression of the rail neck part.

PRIOR ART DOCUMENTS

PATENT DOCUMENTS

[0013]

[Patent Document 1] Japanese Examined Patent Application, Second Publication No. S63-023244

[Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H8-144016

[Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2004-43863

[Patent Document 4] Japanese Patent No. 4819183

[Patent Document 5] Japanese Unexamined Patent Application, First Publication No. H8-170120

[Patent Document 6] Japanese Unexamined Patent Application, First Publication No. 2002-226915

[Patent Document 7] Japanese Unexamined Patent Application, First Publication No. H8-246100

DISCLOSURE OF THE INVENTION

PROBLEMS SOLVED BY THE INVENTION

[0014]

The present invention has been made in view of the above problems. It is an object of the present invention to provide a rail that can suppress fatigue damage arising from a neck portion and has excellent fatigue failure resistance required for a freight rail. In particular, it is an object of the present invention to provide a rail that can suppress the occurrence of fatigue damage even when the rail is used on a curved track where fatigue failure is likely to occur.

REMEDIES FOR SOLVING THE PROBLEM

[0015]

The concept of the present invention is as follows.

(1) A rail according to one aspect of the present invention has a steel composition comprising, in wt %: C: 0.75-1.20%; Si: 0.10-2.00%; Mn: 0.10-2.00%; Cr: 0-2.00%; Mo: 0-0.50%; Co: 0-1.00%; B: 0-0.0050%; Cu: 0-1.00%; Ni: 0-1.00%; V: 0-0.50%; Nb: 0-0.050%; Ti: 0-0.0500%; Mg: 0-0.0200%; Ca: 0-0.0200%; REM: 0-0.0500%; Zr: 0-0.0200%; N: 0-0.0200%; Al: 0-1.00%; P: 0.0250% or less; S: 0.0250% or less; and a residue composed of Fe and impurities, in which 90% of the area or more of the metallographic structure in the cross section of the rail neck part is pearlite structure, the minimum hardness value in the cross section of the rail neck part is 300 or more HV, and the difference between the maximum value and the minimum hardness value in the cross section of the rail neck part is 40 HV or less.

(2) In the rail of (1), the difference between the maximum value and the minimum hardness value in the cross section of the rail neck portion may be 20 HV or less.

(3) In the rail according to (1) or (2), the steel composition may include, in mass%, one, two or more elements selected from the group consisting of: Cr: 0.01-2.00 %; Mo: 0.01-0.50%; Co: 0.01-1.00%; B: 0.0001-0.0050%; Cu: 0.01-1.00%; Ni: 0.01-1.00%; V: 0.005-0.50%; Nb: 0.0010-0.050%; Ti: 0.0030-0.0500%; Mg: 0.0005-0.0200%; Ca: 0.0005-0.0200%; REM: 0.0005-0.0500%; Zr: 0.0001-0.0200%; N: 0.0060-0.0200%; and Al: 0.0100-1.00%.

BENEFICIAL EFFECTS OF THE INVENTION

[0016]

According to an aspect of the present invention, a rail can be provided that has excellent fatigue damage resistance required for a rail neck portion intended for a curved track of a freight railway.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]

Fig. 1 shows the position of the hardness measurement in the cross section of a portion of the rail neck.

Fig. 2 shows a diagram of a rail fatigue test.

Fig. 3 is a graph showing the relationship of the difference between the maximum value and the minimum value of the cross-sectional hardness of the rail neck part and the number of repetitions at the time of crack initiation in the rail fatigue test.

Fig. 4 is a schematic cross-sectional view of a rail according to the present embodiment.

Fig. 5 shows a portion of the neck portion requiring a pearlite structure.

EMBODIMENTS OF THE PRESENT INVENTION

[0018]

Next, a rail having excellent fatigue damage resistance in a neck portion according to one embodiment of the present invention (also referred to as a rail according to the present embodiment) will be described in detail. Hereinafter, "%" in the composition means the mass percentage.

[0019]

First, the inventors of the present invention investigated in more detail the cause of fatigue damage originating from the rail neck part in existing freight railways. As a result of a detailed study of the rail, including the pearlite structure in which fatigue damage occurred, it was found that there is a correlation between the cross-sectional hardness of the neck part and the fatigue damage of the rail. It was confirmed that in a rail in which there was a region where the cross-sectional hardness of the rail neck portion was less than HV 300, fatigue damage occurred from the rail neck portion.

[0020]

In addition, the inventors of the present invention examined a rail in which fatigue damage occurred in more detail. As a result, the case was confirmed that, in a curved track subjected to severe operating conditions, even in a rail in which there was no region in which the cross-sectional hardness of the rail neck portion was less than HV 300, fatigue damage occurred from the neck portion.

[0021]

Thus, the inventors of the present invention made an evaluation of the real rail prototype in order to investigate in more detail the cause of the fatigue damage originating from the neck part even in such a rail in which there is no area in which the cross-sectional hardness of the rail neck part is less than HV 300.

[0022]

Here, the present inventors performed a fatigue damage test to simulate a curved path in prototype evaluation. The reason for this is that there are unique circumstances in which a bending stress can be applied to a portion of a neck in a curved path. As shown in FIG. 4, the rail includes a rail neck part 1, a rail head part 2, and a rail bottom part 3. Since the rail neck part 1 did not come into contact with the wheels, it was not considered important in the prior art. However, in a curved track during the passage of the train, a stress is applied to the railhead portion 2 towards the outside of the curved track, so that a bending stress is applied to the rail neck portion 1. The inventors of the present invention assumed that fatigue damage could occur in part 1 of the rail neck on a curved track due to the repeated application of such a bending stress, and considered that a fatigue damage test should also be performed to reproduce the above described bending stress. Details of the prototype evaluation methodology will be shown below.

[0023]

Hot rolling and heat treatment of the rail were carried out on a steel (hypereutectoid steel) having the following steel composition under various conditions to produce prototype rails having different cross-sectional hardness in rail neck portions, and the prototype rails were evaluated for fatigue damage resistance. Then, the relationship between the cross-sectional hardness of the rail neck portion and fatigue damage resistance was investigated. The following are rail hot rolling conditions, heat treatment conditions, and fatigue damage test conditions. In this regard, in order to change the hardness of the cross section of the rail neck part, controlled cooling of the neck part was performed.

[0024]

[Hot rolling and heat treatment conditions of the actual rail]

Steel composition

0.90% C - 0.50% Si - 0.70% Mn - 0.0150% P - 0.0120% S (with balance of Fe and impurities)

Rail shape

141 lbs (weight: 70 kg/m)

Hot rolling and heat treatment conditions

Final rolling temperature (outer surface of neck part): 900°C.

Heat treatment conditions: hot rolling → accelerated cooling

Controlled cooling conditions (outer surface of the neck part): accelerated cooling in the temperature range from 800°C to 500°C was performed at an average cooling rate of 0.5-5°C/s, or accelerated cooling was performed from 800°C to the temperature range of 580 °C - 680°C. Then, after raising the temperature by generating reheat and maintaining the temperature, accelerated cooling was performed again.

In this regard, accelerated cooling has been carried out by injecting a cooling medium, such as air or cooling water, either onto the surface of the railhead part, or the surface of the neck part, or both. In addition, the rise in temperature by generating reheat and maintaining the temperature was controlled by repeating slightly accelerated cooling depending on the magnitude of the rise in temperature.

[0025]

[Method of measuring the cross-sectional hardness of the rail neck part, measurement conditions and method for determining the hardness]

Device and method of measurement

Device: Vickers hardness tester (load 98 N)

Taking a test sample for measurement: the sample was cut from the cross section of the rail neck part

Pretreatment: The cross section was polished with diamond abrasive particles having an average grain size of 1 µm

Measurement Method: Hardness measured according to JIS Z 2244:2009

Measurement position

The measurement position was in the cross section in the range of ±15 mm in the up-down direction from the center line between the bottom of the rail and the top of the rail (see Fig. 1).

Indentations were continuously made in the thickness direction of the neck portion in a row in 1.0 mm increments, with the starting point of continuous indentation being a position at a depth of 1.0 mm from the outer surface of the neck portion, and the hardness distribution was measured. The hardness measurement was performed on at least five lines.

In this regard, in order to eliminate the mutual influence of the depressions between the measurement lines, an interval of 1.0 mm or more was provided.

Method for determining hardness

The minimum value and maximum value of the measured hardness were determined as the minimum value and maximum value of the hardness of the cross section of each of the parts of the rail neck.

[Test rail hardness specifications]

Range of the minimum value of the cross-sectional hardness of the rail neck part: HV 300-500

Difference between the minimum value and the maximum value of the hardness of the cross section of the rail neck part: HV 10-80

[0026]

[Method and Conditions of Fatigue Resistance Test for Rail Neck Part]

Rail fatigue test

Test Method: Actual rail flexure at three points (spanning length: 650mm, see Fig. 2)

Load mode: fluctuations in the range of 2-20 tons.

Applied load oscillation frequency: 5 Hz

Test position: An eccentric load was applied to the rail head part. The load application position was set to a position offset by one third of the rail head portion width from the center of the rail head portion in the rail width direction (see FIG. 2).

Stress Measurement: Stress was measured with a strain gauge attached to the neck part of the rail

Number of repetitions of load fluctuations: up to a maximum of 3 million repetitions (without crack initiation) or until crack initiation.

Crack detection: the test was periodically stopped and magnetic particle inspection was carried out on the surface of the rail neck part to confirm the presence or absence of a crack on the surface of the rail neck part

Determination of passing: a rail in which the number of repetitions of the load swing before the occurrence of a crack was 2 million or more, or the crack did not form until the end of the test (the number of repetitions of the load swing was 3 million) was determined as a rail having excellent fatigue fracture resistance

[0027]

As shown in FIG. 2, when an eccentric load was applied to the rail, oscillating at regular intervals, a bending stress was applied to the neck part of the rail at regular intervals. Accordingly, it was possible to simulate the bending stress (tensile stress in the up and down direction applied to that side of the rail neck part that corresponds to the outside of the curve) applied to the rail neck part due to the centrifugal force of the composition passing along the curved track.

As a result of a detailed examination of the part of the rail neck in which the crack occurred before the number of repetitions of the load fluctuation reached 2 million, it was confirmed that the crack occurred in the rail in which the hardness in the cross section was significantly non-uniform (namely, the difference between the maximum value and the minimum cross-sectional hardness value was large). The inventors of the present invention, based on this result, found that the occurrence of a crack is the result of strain concentration in the cross section of the neck part due to a large inhomogeneity in the hardness of the cross section.

[0028]

Fig. 3 shows the results of rail fatigue tests. Fig. 3 shows the relationship of the difference between the maximum value and the minimum value of the hardness in the cross section of the rail neck part and the number of repetitions of the load fluctuation before the crack initiation in the fatigue test. As can be seen from the results shown in Fig. 3, there is a correlation between the difference between the maximum value and the minimum value of the cross-sectional hardness and the number of repetitions of the load fluctuation before the crack initiation in the fatigue test, and when the difference between the maximum value and the minimum value of the cross-sectional hardness decreases, the number of repetitions of the load fluctuation before the crack initiation cracks tend to increase. Especially, the present inventors confirmed that when the difference between the maximum value and the minimum value of the cross-sectional hardness was HV 40 or less, the crack did not occur until the number of repetitions of the load fluctuation reached 2 million, and the damage resistance of the neck part was significantly improved.

[0029]

In addition, the present inventors confirmed that when the difference between the maximum value and the minimum value of the cross-sectional hardness of the neck portion was HV 20 or less, the number of repetitions of the load fluctuation until the crack was initiated was further increased, and the crack did not occur up to 3 million repetitions, and the damage resistance of the neck portion has been further improved.

[0030]

Increasing the hardness (full hardening) of a material is said to be effective in preventing fatigue failure of the material. However, the present inventors have recently found that in order to suppress the occurrence of fatigue damage in the rail neck portion, in addition to increasing the hardness of the rail neck portion, it is necessary to suppress the difference between the maximum value and the minimum hardness value in the cross section of the rail neck portion, and also to suppress the stress concentration in cross section of the rail neck.

[0031]

Fig. 4 is a schematic cross-sectional view of a rail according to the present embodiment. The rail neck part (rail neck part 1) according to the present embodiment will be described again with reference to FIG. four.

[0032]

In a cross section perpendicular to the length direction of the rail, a portion of the rail that tapers in width is present at the center in the height direction of the rail. This tapered portion is referred to as part 1 of the rail neck. The part that has a width greater than the width of the necked part and is located below it is referred to as the rail base part 3, and the part located above the narrowed part is referred to as the rail head part 2. The rail neck portion 1 is an area located between the rail head portion 2 and the rail base portion 3.

[0033]

(1) Reasons for limiting the chemical composition (components) of rail steel

Next, the reasons for limiting the chemical composition (components) of the steel in the rail according to the present embodiment will be described in detail.

[0034]

C: 0.75-1.20%

C is an element that promotes pearlite transformation and also helps to improve fatigue resistance. However, when the C content is less than 0.75%, the lower tensile strength or fatigue damage resistance required for the rail cannot be secured. In addition, when the C content is less than 0.75%, a soft pro-eutectoid ferrite structure may be formed in the rail neck portion, the difference in hardness in the cross section of the rail neck portion increases, and fatigue damage resistance deteriorates. On the other hand, when the C content exceeds 1.20%, a hard proeutectoid cementite structure may be formed in the rail neck portion, the difference in hardness in the cross section of the rail neck portion increases, and fatigue damage resistance deteriorates. Therefore, in order to support pearlite structure formation and guarantee fatigue damage resistance, the C content is set to 0.75-1.20%. In order to further stabilize pearlite structure formation and further improve fatigue damage resistance, it is desirable that the C content be 0.80% or more, 0.85% or more, or 0.90% or more. In addition, for the same reason, it is desirable that the C content be 1.15% or less, 1.10% or less, or 1.05% or less.

[0035]

Si: 0.10-2.00%

Si is an element that solidly dissolves in pearlite structure ferrite, increases the cross-sectional hardness (strength) of the rail neck portion, and improves fatigue damage resistance. In addition, Si is also an element that suppresses the formation of the hypoeutectoid cementite structure, suppresses the difference in hardness in the cross section of the rail neck portion, and improves the fatigue damage resistance. However, when the Si content is less than 0.10%, these effects cannot be sufficiently obtained. On the other hand, when the Si content exceeds 2.00%, a large number of surface defects are generated during hot rolling. In addition, when the Si content exceeds 2.00%, the hardenability greatly increases, a hard martensite structure may form in the rail neck portion, the difference in hardness in the cross section of the rail neck portion increases, and fatigue damage resistance deteriorates. Therefore, in order to support pearlite structure formation and ensure fatigue damage resistance or toughness, the Si content is set to 0.10-2.00%. In order to further stabilize pearlite structure formation and further improve fatigue damage resistance or toughness, it is desirable that the Si content be 0.15% or more, 0.20% or more, or 0.40% or more. For the same reason, it is desirable that the Si content be 1.80% or less, 1.50% or less, or 1.30% or less.

[0036]

Mn: 0.10-2.00%

Mn is an element that increases the hardenability, suppresses the formation of the soft structure of proeutectoid ferrite, and stabilizes the pearlite transformation, and at the same time reduces the distance between the lamellar structure of the pearlite structure and ensures its hardness, and thus improves the resistance to fatigue damage. However, when the Mn content is less than 0.10%, this effect decreases, a soft pro-eutectoid ferrite structure may be formed in the rail neck portion, the difference in hardness in the cross section of the rail neck portion increases, and fatigue damage resistance deteriorates. On the other hand, when the Mn content exceeds 2.00%, the hardenability greatly increases, a hard martensite structure may form in the rail neck portion, the difference in hardness in the cross section of the rail neck portion increases, and fatigue damage resistance deteriorates. Therefore, in order to support pearlite structure formation and ensure fatigue damage resistance or toughness, the Mn content is set to 0.10-2.00%. In order to stabilize pearlite structure formation and further improve fatigue damage resistance or toughness, it is desirable that the Mn content be 0.20% or more, 0.30% or more, or 0.40% or more. For the same reason, it is desirable that the Mn content be 1.80% or less, 1.50% or less, or 1.20% or less.

[0037]

P: 0.0250% or less

P is a contaminant element contained in steel. Its content can be controlled by performing cleaning in the converter. It is preferable that the P content be small, but in particular, when the P content exceeds 0.0250%, the P concentration in the segregation zone of the rail neck portion increases, the hardness of the segregation portion increases, the difference in hardness in the cross section of the rail neck portion increases, and the durability deteriorates to fatigue damage. For this reason, the P content is limited to 0.0250% or less. In this regard, in order to stably ensure the fatigue damage resistance of the rail neck portion, it is desirable that the P content be 0.0200% or less, 0.0180% or less, or 0.0150% or less. Since P does not contribute to solving the problem of the present invention, there is no need to limit the lower limit of the P content, and the lower limit can be set to 0%, for example. However, given the potential for dephosphorization in the purification process, it is economically advantageous to set a lower limit for the P content to approximately 0.0050%.

[0038]

S: 0.0250% or less

S is a contaminant element contained in steel. Its content can be controlled by performing desulfurization in a smelter. It is preferable that the S content is small, but in particular, when the S content exceeds 0.0250%, the formation of MnS-based sulfide is accelerated, and the concentration of Mn in the steel is reduced. As a result, a negative segregation portion is formed, the hardness of the negative segregation portion decreases, the difference in hardness in the cross section of the rail neck portion increases, and the fatigue damage resistance deteriorates. For this reason, the S content is limited to 0.0250% or less. In this regard, in order to more stably ensure the fatigue damage resistance of the rail neck portion, it is desirable that the S content be 0.0200% or less, 0.0180% or less, or 0.0150% or less. Since S does not contribute to solving the problem of the present invention, there is no need to limit the lower limit of the S content, and the lower limit can be set to 0%, for example. However, given the possibilities of desulfurization in the purification process, it is economically advantageous to set the lower limit of the S content to approximately 0.0030%.

[0039]

Basically, the rail according to the present embodiment has the above-mentioned chemical composition with the remainder of Fe and impurities. However, instead of a portion of Fe in the remainder, if necessary, further improve the fatigue damage resistance by increasing the hardness (strength) of the pearlite structure, in particular controlling the hardness distribution of the cross section of the rail neck part, one, two or more elements selected from the group consisting of of Cr, Mo, Co, B, Cu, Ni, V, Nb, Ti, Mg, Ca, REM, Zr, N, and Al may be included in ranges to be described later. In particular, Cr and Mo reduce the distance between the lamellar structure of pearlite, improving its hardness. Co refines the lamellar structure, increasing the hardness of the pearlite structure. B reduces the dependence of the pearlite transformation temperature on the cooling rate to evenly distribute the hardness in the cross section of the rail neck. Cu is solid-dissolved in the ferrite of the pearlite structure to increase the hardness of the pearlite structure. Ni is solidly dissolved in the ferrite of the pearlite structure to increase the hardness of the pearlite structure. V, Nb, and Ti improve the hardness of the pearlite structure by precipitation hardening of carbide or nitride formed during hot rolling or during cooling after hot rolling. Mg, Ca, and REM finely disperse the MnS-based sulfide, promoting pearlite transformation. Zr increases the proportion of equiaxed crystals in the solidification structure to suppress the formation of a segregation zone in the central part of a bloom or slab, suppress the formation of a pro-eutectoid ferrite structure or a hypo-eutectoid cementite structure, and promote pearlite transformation. N segregates to the austenite grain boundary, promoting pearlite transformation. Al shifts the eutectoid transformation temperature to higher temperatures to improve the hardness of the pearlite structure. For this reason, in order to obtain the above effects, these elements may be included in ranges to be described later. Therefore, even when these elements are included in the ranges to be described later, these elements do not degrade the performance of the rail according to the present embodiment. In addition, since these elements do not have to be present, the lower limit of their content is set to 0%.

[0040]

Cr: 0-2.00%

Cr raises the equilibrium transformation temperature and increases the degree of supercooling, and thus reduces the distance between the lamellar structures of the pearlite structure and improves the hardness (strength) of the pearlite structure. In addition, Cr is an element that increases hardenability, suppresses the formation of a soft structure of hypoeutectoid ferrite, stabilizes pearlite transformation, and improves fatigue damage resistance. In order to obtain these effects, it is preferable that the Cr content is 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, when the Cr content exceeds 2.00%, the hardenability may increase significantly, a hard martensite structure may form in the rail neck portion, the difference in hardness in the cross section of the rail neck portion may increase, and fatigue damage resistance may deteriorate. For this reason, it is preferable that the Cr content, if present, be 2.00% or less, 1.80% or less, or 1.50% or less.

[0041]

Mo: 0-0.50%

Like Cr, Mo raises the equilibrium transformation temperature and increases the degree of supercooling, and thus reduces the distance between the plates of the pearlite structure and improves the hardness (strength) of the pearlite structure. In particular, Mo is an element that increases the hardness of the soft pearlite structure of the rail neck portion, reduces the difference in hardness in the pearlite structure, and improves the fatigue damage resistance of the rail neck portion. In order to obtain these effects, it is preferable that the Mo content is 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, when the Mo content exceeds 0.50%, the transformation rate is greatly reduced, a hard martensite structure may be formed in the rail neck portion, the difference in hardness in the cross section of the rail neck portion may increase, and fatigue damage resistance may deteriorate. For this reason, it is preferable that the content of Mo, if present, is 0.50% or less, 0.40% or less, or 0.30% or less.

[0042]

Co: 0-1.00%

Co refines the lamellar structure in the pearlite structure to improve the hardness (strength) of the pearlite structure. In particular, Co is an element that increases the hardness of the soft pearlite structure of the rail neck portion, reduces the difference in hardness in the pearlite structure, and improves the fatigue damage resistance of the rail neck portion. In order to obtain these effects, it is preferable that the Co content is 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, when the Co content exceeds 1.00%, the aforementioned effect saturates and economic efficiency may deteriorate due to an increase in the cost of adding alloys. For this reason, it is preferable that the content of Co, if present, is 1.00% or less, 0.80% or less, or 0.50% or less.

[0043]

B: 0-0.0050%

B is an element that forms iron boron carbide (Fe ₂₃ (CB) ₆ ) at the austenite grain boundary and promotes pearlite transformation, and thus reduces the dependency of the pearlite phase transformation temperature on the cooling rate. When the dependence of the pearlite transformation temperature on the cooling rate is reduced, the hardness distribution in the cross section of the rail neck portion becomes more uniform, and the fatigue damage resistance is improved. In order to obtain these effects, it is preferable that the B content is 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, when the B content exceeds 0.0050%, coarse iron boron carbide may be formed, and fatigue damage may occur in the rail neck part due to stress concentration. For this reason, it is preferable that the content of B, if present, be 0.0050% or less, 0.0040% or less, or 0.0030% or less.

[0044]

Cu: 0-1.00%

Cu is solid-dissolved in pearlite structure ferrite to increase hardness (strength) by hardening the solid solution. In particular, Cu is an element that increases the hardness of the soft pearlite structure of the rail neck portion, reduces the difference in hardness in the pearlite structure, and improves the fatigue damage resistance of the rail neck portion. In order to obtain these effects, it is preferable that the Cu content is 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, when the Cu content exceeds 1.00%, due to the greatly improved hardenability, a hard martensite structure may be formed in the rail neck portion, the difference in hardness in the cross section of the rail neck portion may increase, and fatigue damage resistance may deteriorate. For this reason, it is preferable that the content of Cu, if included, is 1.00% or less, 0.80% or less, or 0.50% or less.

[0045]

Ni: 0-1.00%

Ni improves the toughness of the pearlite structure, and at the same time improves the hardness (strength) by solid solution strengthening. In particular, Ni is an element that increases the hardness of the soft pearlite structure of the rail neck portion, reduces the difference in hardness in the pearlite structure, and improves the fatigue damage resistance of the rail neck portion. In order to obtain these effects, it is preferable that the Ni content is 0.01% or more, 0.02% or more, or 0.10% or more. On the other hand, when the Ni content exceeds 1.00%, due to the greatly improved hardenability, a hard martensite structure may be formed in the rail neck portion, the difference in hardness in the cross section of the rail neck portion may increase, and fatigue damage resistance may deteriorate. For this reason, it is preferable that the Ni content, if present, be 1.00% or less, 0.80% or less, or 0.50% or less.

[0046]

V: 0-0.50%

V increases the hardness (strength) of the pearlite structure due to precipitation hardening of V carbides and nitrides formed during the cooling process after hot rolling. In particular, V is an element that increases the hardness of the soft pearlite structure of the rail neck portion, reduces the difference in hardness in the pearlite structure, and improves the fatigue damage resistance of the rail neck portion. In order to obtain these effects, it is preferable that the V content is 0.005% or more, 0.010% or more, or 0.050% or more. On the other hand, when the V content exceeds 0.50%, precipitation hardening due to V carbides or V nitrides may be excessive, the pearlite structure may become brittle, and the fatigue damage resistance of the rail neck portion may deteriorate. For this reason, it is preferable that the content of V, if present, be 0.50% or less, 0.40% or less, or 0.30% or less.

[0047]

Nb: 0-0.050%

Similarly to V, Nb increases the hardness (strength) of the pearlite structure due to the precipitation hardening of Nb carbides and nitrides formed during cooling after hot rolling. In particular, Nb is an element that increases the hardness of the soft pearlite structure of the rail neck portion, reduces the difference in hardness in the pearlite structure, and improves the fatigue damage resistance of the rail neck portion. In order to obtain these effects, it is preferable that the Nb content is 0.0010% or more, 0.0050% or more, or 0.010% or more. On the other hand, when the Nb content exceeds 0.050%, precipitation hardening due to Nb carbides or nitrides may be excessive, the pearlite structure may become brittle, and the fatigue damage resistance of the rail neck portion may deteriorate. For this reason, it is preferable that the Nb content, if present, be 0.050% or less, 0.040% or less, or 0.030% or less.

[0048]

Ti: 0-0.0500%

Ti precipitates as Ti carbide and Ti nitride formed in the cooling process after hot rolling, and increases the hardness (strength) of the pearlite structure by precipitation hardening. In particular, Ti is an element that increases the hardness of the soft pearlite structure of the rail neck portion, reduces the difference in hardness in the pearlite structure, and improves the fatigue damage resistance of the rail neck portion. In order to obtain these effects, it is preferable that the Ti content is 0.0030% or more, 0.0100% or more, or 0.0150% or more. On the other hand, when the Ti content exceeds 0.0500%, coarse Ti carbide and coarse Ti nitride may be formed, and fatigue damage may occur in the rail neck portion due to stress concentration. For this reason, it is preferable that the content of Ti, if present, is 0.0500% or less, 0.0400% or less, or 0.0300% or less.

[0049]

Mg: 0-0.0200%

Mg is an element that bonds with sulfur to form finely dispersed sulfide (MgS). MgS finely disperses MnS. In addition, the finely dispersed MnS serves as the core of pearlite transformation, accelerating it, suppresses the formation of the hypoeutectoid ferrite or hypoeutectoid cementite structure formed in the rail neck part, reduces the difference in hardness in the pearlite structure, and improves the fatigue damage resistance of the rail neck part. In order to obtain these effects, it is preferable that the Mg content is 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, when the Mg content exceeds 0.0200%, coarse Mg oxide may be formed, and fatigue damage may occur in the rail neck part due to stress concentration. For this reason, it is preferable that the Mg content, if present, be 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0050]

Ca: 0-0.0200%

Ca is an element that has a strong bonding strength with sulfur and forms sulfide (CaS). CaS finely disperses MnS. The finely dispersed MnS serves as the core of pearlite transformation, accelerating it, suppresses the formation of the pro-eutectoid ferrite or hypo-eutectoid cementite structure formed in the rail neck part, reduces the difference in hardness in the pearlite structure, and improves the fatigue damage resistance of the rail neck part. In order to obtain these effects, it is preferable that the Ca content is 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, when the Ca content exceeds 0.0200%, coarse Ca oxide may be formed and fatigue damage may occur due to stress concentration. For this reason, it is preferable that the content of Ca, if present, be 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0051]

REM: 0-0.0500%

REM is an element for deoxidation and desulfurization and forms REM oxysulfide (REM ₂ O ₂ S) serving as a core for forming a Mn sulfide-based inclusion when REM is contained. In addition, since the melting point of the oxysulfide (REM ₂ O ₂ S) is high, the elongation of the Mn sulfide-based inclusions after hot rolling is suppressed. As a result, when REM is contained, MnS is finely dispersed and serves as the core of pearlite transformation, and pearlite transformation is accelerated. As a result, the formation of a pro-eutectoid ferrite or hypo-eutectoid cementite structure in the rail neck part is suppressed, the difference in hardness in the pearlite structure is reduced, and the fatigue damage resistance of the rail neck part is improved. In order to obtain these effects, it is preferable that the REM content is 0.0005% or more, 0.0010% or more, or 0.0050% or more. On the other hand, when the REM content exceeds 0.0500%, coarse REM oxysulfide (REM ₂ O ₂ S) may be formed, and fatigue damage may occur in the rail neck portion due to stress concentration. For this reason, it is preferable that the REM content, if present, be 0.0500% or less, 0.0400% or less, or 0.0300% or less.

[0052]

Here, REM is a rare earth metal such as Ce, La, Pr, or Nd. The above content limits the total content of all rare earth metals. When the sum of the amounts of all REM elements is in the above-mentioned range, the same effects can be obtained when REMs are included in the form of a single element or in the form of a plurality of elements.

[0053]

Zr: 0-0.0200%

Zr binds to O and forms ZrO ₂ inclusions. Since the ZrO ₂ inclusions have excellent lattice matching characteristics with γ-Fe, the ZrO ₂ inclusions serve as nuclei for high-carbon rail steel in which γ-Fe is the solidified primary phase, and increase the proportion of equiaxed crystals in the solidification structure, and thus suppress the formation of a segregation zone in of the center part of the bloom or slab, suppress the formation of martensite or hypoeutectoid cementite structure in the rail neck part, reduce the difference in hardness in the pearlite structure, and improve the fatigue damage resistance of the rail neck part. In order to obtain these effects, it is preferable that the Zr content is 0.0001% or more, 0.0010% or more, or 0.0050% or more. On the other hand, when the Zr content exceeds 0.0200%, a large amount of coarse Zr-based inclusions may be formed, and fatigue damage may occur in the rail neck portion due to stress concentration. For this reason, it is preferable that the content of Zr, if present, is 0.0200%, 0.0150%, or 0.0100%.

[0054]

N: 0-0.0200%

N segregates at the austenite grain boundary, and thus promotes pearlite transformation from the austenite grain boundary, suppresses the formation of the proeutectoid ferrite or hypoeutectoid cementite structure in the rail neck portion, reduces the difference in hardness in the pearlite structure, and improves the fatigue damage resistance of the rail neck portion. In addition, when N is included together with V, the precipitation of V carbonitrides in the cooling process after hot rolling is accelerated, the hardness (strength) of the pearlite structure is increased, and the fatigue damage resistance of the rail neck part is improved. In order to obtain these effects, it is preferable that the N content is 0.0060% or more, 0.0080% or more, or 0.0100% or more. On the other hand, when the N content exceeds 0.0200%, it may be difficult to solidly dissolve the N in the steel. In this case, bubbles may form and serve as a source of fatigue damage, and fatigue damage may occur in the neck part of the rail. For this reason, it is preferable that the N content, if present, be 0.0200% or less, 0.0180% or less, or 0.0150% or less.

[0055]

Al: 0-1.00%

Al is an element that functions as a deoxidizer. In addition, Al is an element that shifts the eutectoid transformation temperature upward, and is an element that helps to increase the hardness (strength) of the pearlite structure, increases the hardness of the soft pearlite structure of the rail neck portion, reduces the difference in hardness in the pearlite structure, and improves resistance to fatigue damage of the rail neck part. In order to obtain these effects, it is preferable that the Al content is 0.0100% or more, 0.0500% or more, or 0.1000% or more. On the other hand, when the Al content exceeds 1.00%, it may be difficult to solidify Al in the steel. In this case, coarse alumina-based inclusions may form, fatigue cracks may be initiated from these coarse inclusions, and fatigue damage may occur in the rail neck portion. In addition, in this case, an oxide may be formed during welding of the rail, and the weldability may deteriorate significantly. For this reason, it is preferable that the Al content, if present, be 1.00% or less, 0.80% or less, or 0.60% or less.

[0056]

(2) Metallographic structure

Next, the reason for the limitation in the rail according to the present embodiment of 90% of the area or more of the metallographic structure in the cross section of the neck part by the pearlite structure will be described in detail. In this regard, "metallographic structure in the cross section of the rail neck part" means the metallographic structure in the range of ±15 mm in the up-down direction from the center line between the lower part of the rail and the upper part of the rail in the cross section of the neck part.

[0057]

First, the reason for limiting an area of 90% or more to a pearlite structure will be described.

The pearlite structure is a structure advantageous for improving fatigue damage resistance because strength (hardness) can be easily obtained even if the amount of alloying elements is small. In addition, the strength (hardness) of the pearlite structure can be easily controlled. Therefore, in order to improve the fatigue damage resistance of the cross section of the rail neck portion, the amount of the pearlite structure was limited to a predetermined amount or more.

[0058]

In addition, the area in which the metallographic structure is adjusted in the cross section of the rail neck part is the part that requires fatigue damage resistance in the rail neck part. Fig. 5 shows a portion of the neck portion requiring a pearlite structure. At least 90% of the area or more of the metallographic structure in the range of ±15 mm in the up-down direction from the center line between the lower part of the rail and the upper part of the rail may be a pearlite structure.

[0059]

It is desirable that the metallographic structure in the cross section of the rail neck portion according to the present embodiment be a pearlite structure as described above, but depending on the rail component system or the heat treatment production method, the pro-eutectoid ferrite structure, the pro-eutectoid cementite structure, the bainite structure, or the martensite can be mixed into the pearlite structure in a small amount of 10% or less by area. However, even when these structures are mixed, if the amount of mixing is small, these structures do not significantly affect the hardness of the rail neck portion, and do not significantly adversely affect the fatigue damage resistance of the rail neck portion. For this reason, as the structure of the rail neck part having excellent fatigue damage resistance, it is possible to mix the proeutectoid ferrite structure, the proeutectoid cementite structure, the bainite structure, and the martensite structure in a small amount of 10% area or less. In other words, 90% of the area or more of the metallographic structure in the cross section of the rail neck portion according to the present embodiment may be a pearlite structure. In order to sufficiently improve the fatigue damage resistance, it is desirable that 92% of the area or more, 95% of the area or more, or 98% of the area or more of the metallographic structure in the cross section of the neck part is a pearlite structure.

[0060]

The method for observing and quantifying the structure in the cross section of the rail neck portion is as follows.

[Method for Observing and Quantifying Structure in a Cross-Section of a Part of a Rail Neck]

• Method of observation

Device: optical microscope

Collection of samples for observation: the sample was cut from a cross section in the range of ±15 mm in the up-down direction from the center line between the bottom of the rail and the top of the rail (see Fig. 5).

Pretreatment: The cross section was polished with diamond abrasive with an average grain size of 1 µm, after which nital etching was performed.

Observation Increase: 200

• Surveillance position

Position: position 1.0 mm from the outer surface of the rail neck part and the position of the thickness center of the neck part

• Quantification of structure

Number of observations: five or more fields of view at each of the position 1.0 mm from the outer surface and the position of the center of the thickness of the part of the neck

Quantitative determination: the average value of the area fractions (total 10 or more fields of view) of pearlite at a position of 1.0 mm from the outer surface (five or more fields of view) and at the position of the center of thickness of the part of the neck (five or more fields of view) was determined as the percentage of pearlite in a metallographic structure in the cross section of a part of the rail neck.

[0061]

(3) Reason for limiting the minimum hardness value of the cross section of the neck part

Next, the reason for the restriction in the rail according to the present embodiment of the minimum cross-sectional hardness value of the rail neck part to be in the range of HV 300 or more will be described. In this connection, "minimum hardness value in the cross-section of the neck part of the rail" means the minimum hardness value in the range of ±15 mm in the up-down direction from the center line between the bottom of the rail and the top of the rail in the cross-section of the neck part.

[0062]

When the minimum cross-sectional hardness value of the neck part is less than HV 300, when used in high-load railways, a fatigue crack is initiated from the neck part, the fatigue strength cannot be secured, and the fatigue damage resistance of the rail neck part deteriorates. For this reason, the minimum value of the cross-sectional hardness of the neck part is limited to the range of HV 300 or more. Therefore, in order to stably ensure the fatigue damage resistance of the rail neck part, it is desirable that the minimum cross-sectional hardness value of the neck part is HV 320 or more, HV 340 or more, or HV 360 or more. The maximum value of the cross-sectional hardness value of the neck part is not particularly limited if the requirement for the difference in hardness to be described later is satisfied, but in order to prevent deterioration of the toughness of the rail neck part, it is desirable that the minimum hardness value of the cross-sectional part of the neck be HV 450 or less. , HV 420 or less, or HV 400 or less.

[0063]

(4) Reason for limiting the difference between the maximum value and the minimum hardness value in the cross section of the rail neck part

Next, the reason for the limitation in the rail according to the present embodiment of the difference between the maximum value and the minimum hardness value in the cross section of the rail neck portion of the HV range of 40 or less will be described. In this regard, "the difference between the maximum value and the minimum value of the hardness in the cross-section of the rail neck part" means the difference between the maximum value and the minimum hardness value in the range of ±15 mm in the up-down direction from the center line between the lower part of the rail and the upper part rail in the cross section of the neck part.

[0064]

When the difference between the maximum value and the minimum value of the cross-sectional hardness of the neck portion exceeds HV 40, in heavily loaded railways, the stress applied to the rail neck portion is concentrated in the portion in which the hardness is highly inhomogeneous, and cracks are initiated so that the fatigue damage resistance part of the rail neck is deteriorating. For this reason, the difference between the maximum value and the minimum value of the cross-sectional hardness of the rail neck portion is limited to a range of HV 40 or less.

[0065]

In addition, in order to further improve the fatigue damage resistance of the rail neck portion, it is desirable that the difference between the maximum value and the minimum value of the cross-sectional hardness of the rail neck portion be limited to a range of HV 30 or less, HV 20 or less, or HV 15 or less. Therefore, it is not necessary to limit the value of the lower limit of the difference between the maximum value and the minimum value of the hardness of the cross section of the rail neck part, and the lower limit value can be set to HV 0, but the difference between the maximum value and the minimum value of the hardness of the cross section of the rail neck part usually set to HV 10 or more.

[0066]

The cross-sectional hardness of the rail neck part is measured under the following conditions.

[Method of measuring the cross-sectional hardness of the rail neck part, measurement conditions and method for determining the hardness]

Device and method of measurement

Device: Vickers hardness tester (load 98 N)

Taking a test sample for measurement: the sample was cut from the cross section of the rail neck part

Pretreatment: The cross section was polished with diamond abrasive particles having an average grain size of 1 µm

Measurement Method: Hardness measured according to JIS Z 2244:2009

Measurement position

The measurement position was in the cross section in the range of ±15 mm in the up-down direction from the center line between the bottom of the rail and the top of the rail (see Fig. 1).

Indentations were continuously made in the thickness direction of the neck portion in a row in 1.0 mm increments, with the starting point of continuous indentation being a position at a depth of 1.0 mm from the outer surface of the neck portion, and the hardness was measured. The hardness measurement was performed on at least five lines.

In this regard, in order to eliminate the mutual influence of the depressions between the measurement lines, an interval of 1.0 mm or more was provided.

Method for determining hardness

The minimum value and maximum value of the measured hardness were determined as the minimum value and maximum value of the hardness of the cross section of the rail neck.

[0067]

(5) Method for controlling the cross-sectional hardness of the rail neck part

The cross-sectional hardness of the rail neck portion can be controlled by adjusting, for example, hot rolling conditions and controlled cooling conditions for the head portion and the neck portion after hot rolling.

[0068]

If the rail according to the present embodiment includes the above-described composition, metallographic structures, and hardness, effects can be obtained regardless of the manufacturing method. However, for example, a rail can be obtained by melting a rail steel having the above composition in a commonly used melting furnace such as a converter or an electric furnace, casting liquid steel by an ingot and blooming method or a continuous casting method, performing hot rolling of the obtained bloom or slab. , and performing controlled cooling of the surface of the rail neck portion to control the cross-sectional hardness of the rail neck portion.

[0069]

For example, in the method for producing a rail according to the present embodiment, molten steel after adjusting the composition is cast to form a bloom, and the bloom is heated to 1250-1300°C and hot rolled into a rail shape. Then, the rail according to the present embodiment can be obtained by performing either controlled cooling of the surface of the rail neck portion after hot rolling, or controlled cooling of the surface of the rail neck portion after hot rolling, natural cooling, and then reheating.

[0070]

In a number of processes, operating conditions such as hot rolling conditions, controlled cooling conditions after hot rolling, reheating conditions after hot rolling, and controlled cooling conditions after reheating can be controlled to control the hardness of the cross section of the neck portion. In this regard, the production temperature conditions to be described below should be applied to the entire surface of the rail neck part (outer surface of the rail neck part). Even when these manufacturing temperature conditions are applied to the surface of the railhead part, it is considered that the thermal history, etc. rail neck surfaces are not properly controlled. The rail head portion and the rail neck portion have different thicknesses, and thus have, for example, different reheat rates and the like. during cooling. For this reason, the surfaces of the rail head portion and the rail neck portion inevitably have different thermal histories.

[0071]

Suitable hot rolling conditions and reheating conditions

In order to ensure the cross-sectional hardness of the rail neck part, the final rolling temperature in the neck part is set to 750-1000°C (outer surface temperature of the rail neck part), so that the minimum cross-sectional hardness of the neck part can be ensured.

[0072]

As the hot rolling method, a bloom or slab is subjected to rough rolling by the method described in Patent Document 6, for example, and the like. Thereafter, intermediate rolling is performed in several passes using a reversing mill, and then finishing rolling is performed in two or more passes using a continuous mill, which is a desirable method.

[0073]

In addition, when the rail is temporarily cooled after hot rolling and then subjected to reheating, as reheating conditions, for example, reheating is performed such that the reheating temperature of the rail neck part is in the range of 800-1100°C (temperature outer surface of the rail neck part) so that the minimum cross-sectional hardness of the neck part can be ensured.

[0074]

Suitable controlled cooling conditions after hot rolling and controlled cooling conditions after reheating

The technique for performing controlled cooling of the rail neck portion is not particularly limited. In order to impart resistance to fatigue damage and control cross-sectional hardness, controlled cooling of the rail neck portion is performed during heat treatment using air blowing, mist cooling, water/air mixture blowing cooling, or a combination thereof. In this regard, the cooling rate and the cooling temperature range of the controlled cooling are controlled based on the temperature of the outer surface of the rail neck portion as described above.

[0075]

Controlled cooling is performed in order to increase the uniformity of the cross-sectional hardness of the rail neck portion. There is a segregation zone in the neck portion and the hardness may be uneven. Therefore, in controlled cooling, in order to suppress the increase in hardness of the segregation zone, the accelerated cooling is temporarily stopped after the accelerated cooling of the first stage, the temperature is maintained by using the temperature increase caused by the internal reheating and the slight accelerated cooling, and the increase in hardness of the segregation part is suppressed. In particular, a slight accelerated cooling (controlled cooling) is performed by spraying a cooling medium, so that the increase in the temperature of the outer surface of the neck part caused by reheating and the decrease in the temperature of the outer surface of the neck part due to the spraying of the coolant are balanced so that the temperature of the outer surface of the neck part remains essentially constant. After the end of the holding temperature, accelerated cooling of the second stage is carried out in order to guarantee the hardness. A suitable range of cooling conditions is shown below. Incidentally, the average accelerated cooling rate is a value obtained by averaging the cooling rate at the time of coolant spraying, namely by dividing the difference between the coolant spray start temperature and the coolant spray end temperature by the coolant spray time.

[0076]

(1) When controlled cooling is performed after hot rolling

Controlled part: outer surface of the rail neck part

Average cooling rate during accelerated cooling of the first stage: 0.5-5.0°C/s

Refrigeration stop temperature range: 580-680°C

Temperature hold: 20-200 s in the range of 580-680°C (slightly accelerated cooling)

Average cooling rate during accelerated cooling of the second stage: 2.0-5.0°C/s

Refrigeration stop temperature range: 500°C or less

[0077]

(2) When controlled cooling is performed after reheating

Controlled part: outer surface of the rail neck part

Average cooling rate during accelerated cooling of the first stage: 1.0-6.0°C/s

Refrigeration stop temperature range: 580-680°C

Temperature hold: 20-200 s in the range of 580-680°C (slightly accelerated cooling)

Average cooling rate during accelerated cooling of the second stage: 2.0-5.0°C/s

Refrigeration stop temperature range: 500°C or less

[0078]

Controlled cooling of the neck portion was performed by spraying a cooling medium such as air or cooling water onto either the surface of the rail neck portion, or the surface of the head portion, or both. In addition, the holding temperature can be controlled by repeating a slight accelerated cooling depending on the amount of temperature rise due to the reheating.

In this regard, the part that undergoes controlled cooling is the rail neck part, but when the rail is cooled in a standing position (the head is on top), the cooling medium can be injected onto the surface of the rail head part and flow down to the surface of the rail neck part, cooling it. . Therefore, as described above, direct cooling of the surface of the rail neck portion is not necessary. However, it goes without saying that even when the cooling medium is injected onto the surface of the rail head part, the target of the regulation is the temperature of the outer surface of the neck part.

[0079]

Suitable materials and production conditions for the head part and foot part of the rail

The material of the head part and the bottom part of the rail is not particularly limited. It is desirable to have a structure in which, even with a small amount of alloying elements, strength (hardness) and wear resistance or fatigue damage resistance can be easily obtained.

It is desirable that the rail head part has a pearlite structure with a hardness of HV 340 or more to ensure wear resistance.

It is desirable that the rail bottom portion also has a metallographic structure having a hardness of HV 300 or more to ensure fatigue damage resistance. Since it is not necessary to ensure wear resistance in the outsole part, the outsole part is not limited to a pearlite structure, and may include a metallographic structure such as bainite having an excellent balance between toughness and ductility.

In addition, in order to ensure hardness, it is desirable to heat treat the rail head portion after hot rolling or reheating. The hardness of the rail head portion can be ensured by performing accelerated cooling using the methods described in Patent Document 1, Patent Document 7, and the like. In order to ensure the hardness of the rail foot part, achieve a balance between the rail foot part and the head part during heat treatment, and suppress bending, it is desirable to perform the same accelerated cooling as that of the rail head part.

[0080]

The rail according to the present embodiment can be produced by combining and using the hardness control method of the rail head portion and the novel patterns obtained by the present inventors.

[Examples]

[0081]

Next, examples of the present invention will be described.

Table 1 shows the chemical compositions (steel compositions) of the rails in the examples of the present invention. In Table 1, the remainder of the chemical composition is iron and impurities, and the amount of an element that is not intentionally added is indicated as "-".

Table 3 shows the proportion of perlite (area %) in the cross section of the neck portion, the minimum value (HV) of the hardness of the cross section of the neck portion, and the difference (HV) between the maximum value and the minimum value of the hardness of the cross section of the neck portion. In addition, Table 3 also shows the results of fatigue tests performed using the method shown in FIG. 2. When the proportion of pearlite in the cross section of the neck part is described as 90%, the area ratio of the pearlite structure in the cross section of the rail neck part is 90%, and one, two or more of the hypereutectoid ferrite structure, hypoeutectoid cementite structure, bainite structure and martensite structure are mixed in a small amount, accounting for a total of 10% of the area share.

[0082]

On the other hand, Table 2 shows the chemical compositions of rails in comparative examples. In Table 2, the remainder of the chemical composition is iron and impurities, and the amount of an element that is not intentionally added is indicated as "-".

Table 4 shows the proportion of perlite (area %) in the cross section of the neck portion, the minimum value (HV) of the hardness of the cross section of the neck portion, and the difference (HV) between the maximum value and the minimum value of the hardness of the cross section of the neck portion. In addition, Table 4 also shows the results of fatigue tests performed using the method shown in FIG. 2. When the proportion of pearlite in the cross section of the neck part is described as 86%, the area ratio of the pearlite structure in the cross section of the rail neck part is 86%, and one, two or more of the hypereutectoid ferrite structure, hypoeutectoid cementite structure, bainite structure and martensite structure are mixed in small numbers, accounting for a total of 14% of the area.

[0083]

In this regard, the scheme of manufacturing processes and manufacturing conditions for the rails of the present invention and the comparative rails shown in Tables 1-4 is described below.

[0084]

[Manufacturing Process of the Rail of the Present Invention]

Basic conditions (direct controlled cooling is performed without cooling and reheating after hot rolling)

Molten steel -> composition control -> casting (bloom) -> reheating (1250-1300°C) -> hot rolling -> controlled cooling

Reheat Conditions

Molten steel -> composition control -> casting -> reheating -> hot rolling -> natural cooling -> reheating (rails) -> controlled cooling

[0085]

In addition, a diagram of the production conditions for the rails of the present invention listed in Tables 1 and 3 is shown below. With regard to the production conditions for the comparative rails shown in Tables 2 and 4, Comparative Examples D to K were produced under the basic conditions (controlled cooling after hot rolling) for the rails of the present invention, and Comparative Examples A to C were produced under conditions in which one condition deviated from the production conditions for the rails of the present invention.

[0086]

[Production Conditions for the Rails of the Present Invention]

Basic conditions (controlled cooling after hot rolling)

hot rolling conditions

Controlled part: outer surface of the rail neck part

Final rolling temperature: 750-1000°C

Controlled Cooling Conditions

Controlled part: outer surface of the rail neck part

Average cooling rate during accelerated cooling of the first stage: 0.5-5.0°C/s

Refrigeration stop temperature range: 580-680°C

Temperature hold: 20-200 s in the range of 580-680°C (slightly accelerated cooling)

Average cooling rate during accelerated cooling of the second stage: 2.0-5.0°C/s

Refrigeration stop temperature range: 500°C or less

Reheat conditions (controlled cooling after reheat)

Heating conditions

Controlled part: outer surface of the rail neck part

Heating temperature 800-1100°C

Controlled Cooling Conditions

Controlled part: outer surface of the rail neck part

Average cooling rate during accelerated cooling of the first stage: 1.0-6.0°C/s

Refrigeration stop temperature range: 580-680°C

Exposure at temperature: 20-200 s in the range of 580-680°C

(slightly accelerated cooling)

Average cooling rate during accelerated cooling of the second stage: 2.0-5.0°C/s

Refrigeration stop temperature range: 500°C or less

[0087]

In this regard, the details of the rails of the present invention and the comparative rails listed in Tables 1-4 are shown below.

[0088]

(1) Rails of the present invention (37 samples)

Inventive Examples 1 to 37 were rails in which the values of the chemical composition, the proportion of pearlite in the cross section of the neck portion, the minimum hardness value of the cross section of the neck portion, and the difference between the maximum value and the minimum hardness value of the cross section of the neck portion were within the ranges described in the present invention.

Inventive Examples 1-18 and 23-37 were rails produced under basic conditions (direct controlled cooling was performed after hot rolling), and Inventive Examples 19-22 were rails produced under reheating conditions.

[0089]

(2) Comparative rails (11 samples)

Comparative Examples A to C (3 samples) were rails in which one of the proportion of pearlite in the cross section of the neck portion, the minimum value of the hardness of the cross section of the neck portion, and the difference between the maximum value and the minimum value of the hardness of the cross section of the neck portion was outside the ranges described in the present invention.

With regard to the production conditions of the rail in Comparative Example A, the average cooling rate in the accelerated cooling of the first stage was 0.2°C/s, and other conditions were the same as for the rails of the present invention. As for the production conditions of the rail in Comparative Example B, the final hot rolling temperature was 700°C, and other conditions were the same as for the rails of the present invention. As for the production conditions of the rail in Comparative Example C, the temperature holding time was 10 seconds, and other conditions were the same as for the rails of the present invention. In addition, all rails in Comparative Examples A to C were subjected to direct controlled cooling after hot rolling.

Comparative examples D - K (8 samples) were rails in which the content of one element of C, Si, Mn, P and S was outside the ranges described in the present invention. All rails in Comparative Examples D-K were subjected to direct controlled cooling after hot rolling.

[0090]

A method for observing the structure in a cross section of a rail neck portion is described below.

[Method of Observing Structure in Cross Section of Rail Neck Part]

Observation method

Device: optical microscope

Collection of samples for observation: the sample was cut from a cross section in the range of ±15 mm in the up-down direction from the center line between the bottom of the rail and the top of the rail (see Fig. 5).

Pretreatment: The cross section was polished with diamond abrasive with an average grain size of 1 µm, after which nital etching was performed.

Observation Increase: 200

Observation Position

Position: position 1.0 mm from the outer surface of the rail neck part and the position of the thickness center of the neck part

Structure quantification

Number of observations: five or more fields of view at each of the position 1.0 mm from the outer surface of the rail neck part and the position of the thickness center of the neck part

Quantitative determination: the average value of the area fractions (total 10 or more fields of view) of pearlite at a position of 1.0 mm from the outer surface of the rail neck part (four or more fields of view) and at the position of the center of the thickness of the neck part (five or more fields of view) was determined as the proportion of perlite in the metallographic structure in the cross section of the rail neck part.

[0091]

The method for measuring the cross-sectional hardness of the rail neck part and the measurement conditions are described below.

[Method of measuring the cross-sectional hardness of the rail neck part, measurement conditions and method for determining the hardness]

Device and method of measurement

Device: Vickers hardness tester (load 98 N)

Taking a test sample for measurement: the sample was cut from the cross section of the rail neck part

Pretreatment: The cross section was polished with diamond abrasive particles having an average grain size of 1 µm

Measurement Method: Hardness measured according to JIS Z 2244:2009

Measurement position

The measurement position was in the cross section in the range of ±15 mm in the up-down direction from the center line between the bottom of the rail and the top of the rail (see Fig. 1).

Indentations were continuously made in the thickness direction of the neck portion in a row in 1.0 mm increments, with the starting point of continuous indentation being a position at a depth of 1.0 mm from the outer surface of the neck portion, and the hardness was measured. The hardness measurement was performed on at least five lines.

In this regard, in order to eliminate the mutual influence of the depressions between the measurement lines, an interval of 1.0 mm or more was provided.

Method for determining hardness

The minimum value and maximum value of the measured hardness were determined as the minimum value and maximum value of the hardness of the cross section of the rail neck.

[0092]

In addition to this, the rail fatigue test conditions are described below.

[Rail fatigue test (see Fig. 2)]

Test method: actual rail bending at three points (spanning length: 650mm)

Load mode: fluctuations in the range of 2-20 tons.

Applied load oscillation frequency: 5 Hz

Test position: An eccentric load was applied to the rail head part. The load application position was set to a position offset by one third of the width of the rail head part from the center of the rail head part in the rail width direction (see Fig. 2) (tensile stress was applied to the rail neck part in order to reproduce a curved path).

Stress Measurement: Stress was measured with a strain gauge attached to the neck part of the rail

Number of repetitions of load fluctuations: up to a maximum of 3 million repetitions (without crack initiation) or until crack initiation.

Crack detection: the test was periodically stopped and magnetic particle inspection was carried out on the surface of the rail neck part to confirm the presence or absence of a crack on the surface of the rail neck part

Determination of passing: a rail in which the number of repetitions of the load swing before the occurrence of a crack was 2 million or more, or the crack did not form until the end of the test (the number of repetitions of the load swing was 3 million) was determined as a rail having excellent fatigue fracture resistance

[0093]

The test results were evaluated as follows.

Acceptable material

Grade S: no cracks appeared up to 3 million repetitions at the end of the test.

Grade A: The number of repetitions at the time of the appearance of the crack was 2.5 million or more and less than 3 million.

Evaluation B: the number of repetitions at the time of the appearance of the crack was 2.3 million or more and less than 2.5 million.

Grade C: The number of repetitions at the time of the appearance of the crack was 2 million or more and less than 2.3 million.

Unacceptable material

Score X: The number of repetitions at the time of the appearance of the crack was less than 2 million.

The evaluation results of the examples of the present invention are shown in Table 3, and the evaluation results of the comparative examples are shown in Table 4.

[0094]

[Table 1]

C Si Mn P S Cr Mo co B Cu Ni V Nb Ti mg Ca REM Zr N Al one Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - 2 Example according to the invention 0.80 0.40 1.20 0.015 0.012 - - - - - - - - - - - - - - - 3 Example according to the invention 1.00 0.60 0.85 0.015 0.012 - - - - - - - - - - - - - - - four Example according to the invention 1.10 1.00 0.45 0.015 0.012 - - - - - - - - - - - - - - - 5 Example according to the invention 1.20 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - 6 Example according to the invention 0.75 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - 7 Example according to the invention 0.90 2.00 0.70 0.015 0.012 - - - - - - - - - - - - - - - eight Example according to the invention 0.90 0.10 0.70 0.015 0.012 - - - - - - - - - - - - - - - 9 Example according to the invention 0.90 0.50 2.00 0.015 0.012 - - - - - - - - - - - - - - - ten Example according to the invention 0.90 0.50 0.10 0.015 0.012 - - - - - - - - - - - - - - - eleven Example according to the invention 0.90 0.50 0.70 0.025 0.012 - - - - - - - - - - - - - - - 12 Example according to the invention 0.90 0.50 0.70 0.015 0.025 - - - - - - - - - - - - - - - 13 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - fourteen Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - fifteen Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - 16 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - 17 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - eighteen Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - 19 Example according to the invention 1.00 0.60 0.85 0.015 0.012 - - - - - - - - - - - - - - - twenty Example according to the invention 1.00 0.60 0.85 0.015 0.012 - - - - - - - - - - - - - - - 21 Example according to the invention 1.00 0.60 0.85 0.015 0.012 - - - - - - - - - - - - - - - 22 Example according to the invention 1.00 0.60 0.85 0.015 0.012 - - - - - - - - - - - - - - - 23 Example according to the invention 0.90 0.50 0.70 0.015 0.012 2.00 - - - - - - - - - - - - - - 24 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - 0.50 - - - - - - - - - - - - - 25 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - 1.00 - - - - - - - - - - - - 26 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - 0.0050 - - - - - - - - - - - 27 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - 1.00 - - - - - - - - - - 28 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - 1.00 - - - - - - - - - 29 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - 0.500 - - - - - - - - thirty Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - 0.050 - - - - - - - 31 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - 0.050 - - - - - - 32 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - 0.020 - - - - - 33 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - 0.020 - - - - 34 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - 0.050 - - - 35 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - 0.020 - - 36 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - 0.020 - 37 Example according to the invention 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - 1,000

[0095]

[Table 2]

C Si Mn P S Cr Mo co B Cu Ni V Nb Ti mg Ca REM Zr N Al A Comparative Example 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - B Comparative Example 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - C Comparative Example 0.90 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - D Comparative Example 1.24 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - E Comparative Example 0.71 0.50 0.70 0.015 0.012 - - - - - - - - - - - - - - - F Comparative Example 0.90 2.04 0.70 0.015 0.012 - - - - - - - - - - - - - - - G Comparative Example 0.90 0.06 0.70 0.015 0.012 - - - - - - - - - - - - - - - H Comparative Example 0.90 0.50 2.04 0.015 0.012 - - - - - - - - - - - - - - - I Comparative Example 0.90 0.50 0.06 0.015 0.012 - - - - - - - - - - - - - - - J Comparative Example 0.90 0.50 0.70 0.030 0.012 - - - - - - - - - - - - - - - K Comparative Example 0.90 0.50 0.70 0.015 0.029 - - - - - - - - - - - - - - -

[0096]

[Table 3]

The proportion of pearlite in the cross section of the neck (%) Minimum value of hardness in the cross section of the neck part (HV) Difference between the maximum value and the minimum value of the hardness of the cross section of the neck part (HV) Fatigue Damage Property Evaluation one 96 350 32 B 2 96 320 32 B 3 95 370 33 B four 95 380 34 B 5 93 430 36 C 6 92 340 36 C 7 92 410 37 C eight 92 340 38 C 9 91 440 39 C ten 92 340 37 C eleven 96 345 39 C 12 96 345 37 C 13 90 345 36 C fourteen 96 300 32 C fifteen 96 355 40 C 16 96 345 twenty S 17 96 340 fifteen S eighteen 96 340 ten S 19 95 370 33 B twenty 95 365 eighteen S 21 95 360 13 S 22 95 360 ten S 23 96 440 19 S 24 96 420 25 A 25 96 360 28 A 26 96 340 fifteen S 27 96 410 27 A 28 96 395 24 A 29 96 375 27 A thirty 96 380 25 A 31 96 400 26 A 32 97 350 13 S 33 99 355 16 S 34 96 345 28 A 35 98 360 17 S 36 98 360 16 S 37 98 380 23 A

[0097]

[Table 4]

The proportion of pearlite in the cross section of the neck (%) Minimum value of hardness in the cross section of the neck part (HV) Difference between the maximum value and the minimum value of the hardness of the cross section of the neck part (HV) Fatigue Damage Property Evaluation A 86 280 fifty X B 96 295 31 X C 96 345 45 X D 80 350 fifty X E 85 270 48 X F 75 480 70 X G 85 320 55 X H 78 460 85 X I 85 285 fifty X J 96 370 48 X K 96 345 45 X

[0098]

As shown in Tables 1 to 4, the rails of the present invention (Invention Examples 1 to 37) were evaluated as rails in which occurrence of fatigue damage in the neck portion is suppressed and excellent in fatigue fracture resistance.

In particular, in the rails of the present invention (Invention Examples 1-12), the content of C, Si, Mn, P and S in the steel was more favorable in limited ranges, and the proportion of pearlite in the cross section of the neck part, the minimum hardness value of the cross section of the part The neck portion and the difference between the maximum value and the minimum value of the cross-sectional hardness of the neck portion were further controlled compared to comparative rails (Comparative Examples D to K), so that the fatigue strength of the rail neck portion was improved and the fatigue damage resistance of the rail was improved.

In addition, in the rails of the present invention (Inventive Examples 13 to 22), compared with the comparative rails (Comparative Examples A to C), the hot rolling conditions and the heat treatment conditions of the rail neck portion were controlled more appropriately to control the proportion of pearlite in the cross section of the rail neck portion. of the neck portion, the minimum hardness value of the cross section of the neck portion, and the difference between the maximum value and the minimum hardness value of the cross section of the neck portion, so that the fatigue strength of the neck portion of the rail has been improved, and the fatigue damage resistance of the rail has been improved.

In addition, in the rails of the present invention (Invention Examples 16-18 and 20-22), the controlled cooling conditions for the rail neck portion were further suitably controlled so as to further reduce the difference between the maximum value and the minimum hardness value of the cross section of the neck portion. . As a result, the fatigue strength of the rail neck portion was improved, and the fatigue damage resistance of the rail was further improved.

[0099]

On the other hand, in the rails in Comparative Examples A to K, one or more of the chemical composition, the metallographic structure in the cross section of the rail neck part, the minimum hardness value in the cross section of the rail neck part, and the difference between the maximum value and the minimum hardness value in the cross section of the rail neck part rail necks were inadequate and fatigue damage resistance was degraded.

INDUSTRIAL APPLICABILITY

[0100]

According to the present invention, a rail can be provided in which the composition of the rail steel and the metallographic structure of the rail neck part, and the minimum hardness value of the rail neck part and the difference between the maximum value and the minimum hardness value in the cross section of the rail neck part are suppressed, so that the stress concentration in the cross section of the rail neck portion is suppressed, and the fatigue damage resistance required for the rail neck portion used in the curved tracks of freight railways is excellent.

BRIEF DESCRIPTION OF REFERENCE SYMBOLS

[0101]

1: Part of the rail neck

2: Rail head part

3: Part of the sole of the rail.

Claims (44)

1. Rail containing the composition of steel, including, in wt.%: C: 0.75-1.20; Si: 0.10-2.00; Mn: 0.10-2.00; Cr: 0-2.00; Mo: 0-0.50; Co: 0-1.00; B: 0-0.0050; Cu: 0-1.00; Ni: 0-1.00; V: 0-0.50; Nb: 0-0.050; Ti: 0-0.0500; Mg: 0-0.0200; Ca: 0-0.0200; REM: 0-0.0500; Zr: 0-0.0200; N: 0-0.0200; Al: 0-1.00; P: 0.0250 or less; S: 0.0250 or less; and residue from iron and impurities, in which 90% of the area or more of the metallographic structure in the cross section of the rail neck part is a pearlite structure, the minimum hardness value in the cross section of the rail neck part is 300 or more HV, the difference between the maximum value and the minimum hardness value in the cross section of the rail neck part is 40 or less HV, moreover, the specified cross-section of the rail neck part represents an area in the range of +/-15 mm in the up-down direction of the rail from the center line between the upper part of the rail and the lower part of the rail, wherein said hardness was measured with a Vickers hardness tester with a load of 98 N, while the recesses were continuously made in the direction of the thickness of the rail neck part in a row with a step of 1.0 mm, while the starting point of continuous indentation is the position at a depth of 1.0 mm from the outer surface of the neck part rail, and the hardness measurement was performed on at least five lines. 2. The rail according to claim 1, wherein the difference between the maximum value and the minimum hardness value in the cross section of the rail neck portion is 20 HV or less. 3. The rail according to claim. 1 or 2, in which the composition of the steel includes, in wt.%, one, two or more elements selected from the group consisting of: Cr: 0.01-2.00; Mo: 0.01-0.50; Co: 0.01-1.00; B: 0.0001-0.0050; Cu: 0.01-1.00; Ni: 0.01-1.00; V: 0.005-0.50; Nb: 0.0010-0.050; Ti: 0.0030-0.0500; Mg: 0.0005-0.0200; Ca: 0.0005-0.0200; REM: 0.0005-0.0500; Zr: 0.0001-0.0200; N: 0.0060-0.0200; and Al: 0.0100-1.00.

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